A DC-DC converter, also known as a power converter, refers to an electric circuit that converts a direct current or voltage fed to the input side into a direct current or voltage having a higher, lower or inverted voltage level. DC-DC converters can be found, among others, not only in switched-mode power supply units of PC power supply packs, notebooks, mobile phones and HiFi devices, but also as voltage conditioners in motor drive systems, maximum power point (MPP) trackers in PV installations and battery chargers Their advantages in comparison to linear power supply units are their higher efficiency and lower heat generation. In a linear voltage regulator or in a series resistor, in contrast, the superfluous voltage simply “burns off”.
DC-DC converters are also available as completely encapsulated converter modules that are sometimes intended for direct insertion into printed circuit boards. The output voltage can be lower than, equal to or greater than the input voltage, depending on the model. The best-known modules are the ones that transform a low voltage into a galvanically isolated extra-low voltage. The encapsulated DC-DC converters are available, for example, for insulation voltages ranging from 1.5 kV to over 3 kV, and they serve to supply power to small consumers in direct-voltage networks such as, for example, 24 V in industrial installations or 48 V in telecommunications or in the realm of electronic modules such as, for instance, 5 V for digital circuits or ±15 V for the operation of operational amplifiers. DC-DC converters can also be found in high-power applications, such as automotive and traction. In automotive applications, for example, they serve to charge batteries or to supply power from the batteries or fuel cells to the dc-link of the traction inverter and the on-board low-voltage power supply. DC-DC converters are classified according to various criteria and divided into many different topologies (such a hard-switched, resonant, transition resonant, galvanic isolated, unidirectional, multi-phase, etc. types). In contrast to unidirectional converters, when it comes to bidirectional, or multidirectional, multi-port DC-DC converters, it is immaterial which terminal(s) is defined as the input and which terminal(s) is defined as the output. A bidirectional energy flow allows power to flow from the defined input (primary side) towards the output (secondary side) and vice versa. In the case of multi-port active bridge converters, the ports are not called primary or secondary side, but are instead numbered, e.g. port 1, port 2, port 3 etc.
In DC-DC converters that are based on the functional principle of a bidirectional two-port active bridge converter, the so-called dual active bridge (DAB) topology, the DC input voltage is converted by an input converter into an AC voltage, which is then fed to a transformer or inductor(s). Transformers are used to provide galvanic isolation between the DC ports. The output of the transformer is connected to an output converter that once again converts the AC voltage into a DC output voltage for a load. These DC-DC converters can be implemented in single-phase or multi-phase configurations. Such DAB DC-DC converter topologies as disclosed, for example, in U.S. Pat. No. 5,027,264, constitute high-efficiency converter topologies that allow a bidirectional energy flow and galvanic separation via the transformer and operation at high voltages. This type of converter is particularly well suited for use in high power DC networks, e.g. medium-voltage and low-voltage DC grids and DC collectors for windfarms and large PV installations. Since the transferred power in medium and low-voltage grids is not constant, the need of power flow control and a variable voltage ratio arises. The DAB has the highest efficiency while operating in zero-voltage soft-switching (ZVS) mode under high-power conditions. For low-power operation however, the soft-switching boundary can be violated. To exploit fully renewable energy sources under all weather conditions, operation of the DC-DC converter under every load condition at high-efficiency is needed.
The three-phase DAB DC-DC converter may include at least two actively switched three-phase bridges (phase legs) linked by a three-phase transformer, which may be connected in a star-star (Y-Y) configuration. In order to operate under zero-voltage soft-switching conditions, snubber or resonant capacitors can be placed across the semiconductor devices. However, when the DAB is operating under partial load conditions the magnetically stored energy in the transformer inductance may not be sufficient to fully commutate the voltage of the capacitors. Hard-switching events also occur when the load current remains freewheeling in a diode prior to switching off the anti-parallel active switch while turning on the other switch in the inverter phase leg. The semiconductor devices can even be destroyed when the energy stored in the capacitors is discharged in the devices during the hard-switched turn-on process. To improve the commutation process auxiliary circuits can be implemented to provide a boost current that ensures full commutation of the capacitor voltages. However, auxiliary circuits in the state of the art not only have to commutate the current from the diode to the active switch and provide a boost current to commutate the capacitor voltage but also needs to compensate its losses. Therefore, the boost current needs to be accurately controlled and measured to guarantee safe ZVS operation. Hence, the state of the art circuits require fast switches, high-bandwidth and precise boost current control, which makes said circuitry very expensive.
Therefore, it is desirable to provide means for a DC-DC converter to operate in zero-voltage soft-switching (ZVS) mode under all load conditions and to overcome the disadvantages of the state of the art.